Topical microbicides are an important strategy to minimize heterosexual transmission of HIV. Several single agent microbicides are in clinical trials, including one based on the nonnucleoside reverse transcriptase inhibitor (NNRTI) UC781 that we discovered as a potential microbicidal agent. However, combination microbicides may be preferable, yet only a single combination microbicide is currently under evaluation. There is also an urgent need to identify new pipeline microbicidal agents. We have found that the nucleoside RT inhibitor (NRTI) 4'-ethynyl-2-fluoro-deoxyadenosine (4'E-2FdA) provides a potent and prolonged barrier to HIV- 1 infection of cells in the subsequent absence of exogenous drug, a property previously only noted for NNRTI such as UC781. The "memory effect" barrier is imparted by 4'E-2FdA at drug levels orders of magnitude less than those needed for protection by the nucleotide tenofovir, currently in clinical assessment for microbicide use. We hypothesize that microbicides comprising combinations of different classes of highly potent RT inhibitors, namely the NNRTI UC781 and an NRTI such as 4'E-2FdA, will provide an optimal barrier to HIV-1 transmission. We therefore propose these Specific Aims for this R21/R33 phased innovation application: R21 Aim 1. To evaluate the in vitro (cell-based) microbicidal properties of NRTI and UC781 alone and in combination. These studies include assessment of antiviral activity and "memory effect" protection imparted by NRTIs and UC781 alone and in combination using primary cells (PBMCs, CD4+ T-cells, macrophages) and different HIV drug-sensitive and drug-resistant strains, isolates and clades. R21 Aim 2. To elucidate the mechanism of 4'E-2FdA (and analogs) induced protective barrier or "memory effect" in HIV susceptible cells. These studies include characterization of uptake, conversion to triphosphate and intracellular stability of the NRTI-TPs, as well as detailed kinetic evaluations of the NRTI substrate activity with enzymes involved in metabolism of the NRTIs. R21 Deliverables: Identification of a lead NRTI and two back-ups for use with UC781 for development as a combination microbicide. R33 Aim 1. To formulate the NRTI/NNRTI combinations selected in the R21 phase into an appropriate delivery system for vaginal topical use. NRTIs and NNRTIs have different chemical properties, thus appropriate delivery systems must be identified to enable incorporation and release of the active agents. We will prepare and evaluate both gel and rapidly dissolving film formulations for the combination microbicide. R33 Aim 2. To evaluate the anti-HIV microbicidal activity of formulated NRTI/NNRTI combinations in an ex vivo cervical explant tissue model. These studies will use a newly developed physiologically relevant polarized cervical tissue model to assess the impact of formulated microbicides alone and in combination on HIV transmission and infectivity. R33 Deliverables: Identification of an appropriate delivery formulation for the selected NRTI/NNRTI combination for entry into subsequent preclinical safety and efficacy studies.